MIC2582(2003) Ver la hoja de datos (PDF) - Micrel

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componentes Descripción

Fabricante

MIC2582
(Rev.:2003)

Single Channel Hot Swap Controllers

Micrel 

MIC2582/MIC2583

MOSFET Steady-State Thermal Issues

The selection of a MOSFET to meet the maximum continuous

current is a fairly straightforward exercise. First, arm yourself

with the following data:

• The value of ILOAD(CONT, MAX.) for the output in

question (see Sense Resistor Selection).

• The manufacturer’s data sheet for the candidate

MOSFET.

• The maximum ambient temperature in which the

device will be required to operate.

• Any knowledge you can get about the heat

sinking available to the device (e.g., can heat be

dissipated into the ground plane or power plane,

if using a surface-mount part? Is any airflow

available?).

The data sheet will almost always give a value of on resis-

tance given for the MOSFET at a gate-source voltage of 4.5V,

and another value at a gate-source voltage of 10V. As a first

approximation, add the two values together and divide by two

to get the on-resistance of the part with 8V of enhancement.

Call this value RON. Since a heavily enhanced MOSFET acts

as an ohmic (resistive) device, almost all that’s required to

determine steady-state power dissipation is to calculate I2R.

The one addendum to this is that MOSFETs have a slight

increase in RON with increasing die temperature. A good

approximation for this value is 0.5% increase in RON per °C

rise in junction temperature above the point at which RON was

initially specified by the manufacturer. For instance, if the

selected MOSFET has a calculated RON of 10mΩ at a

TJ = 25°C, and the actual junction temperature ends up

at 110°C, a good first cut at the operating value for RON

would be:

RON ≅ 10mΩ[1 + (110 - 25)(0.005)] ≅ 14.3mΩ

The final step is to make sure that the heat sinking available

to the MOSFET is capable of dissipating at least as much

power (rated in °C/W) as that with which the MOSFET’s

performance was specified by the manufacturer. Here are a

few practical tips:

1. The heat from a surface-mount device such as

an SO-8 MOSFET flows almost entirely out of

the drain leads. If the drain leads can be sol-

dered down to one square inch or more, the

copper will act as the heat sink for the part. This

copper must be on the same layer of the board

as the MOSFET drain.

2. Airflow works. Even a few LFM (linear feet per

minute) of air will cool a MOSFET down sub-

stantially. If you can, position the MOSFET(s)

near the inlet of a power supply’s fan, or the

outlet of a processor’s cooling fan.

3. The best test of a surface-mount MOSFET for

an application (assuming the above tips show it

to be a likely fit) is an empirical one. Check the

MOSFET's temperature in the actual layout of

the expected final circuit, at full operating

Micrel

current. The use of a thermocouple on the drain

leads, or infrared pyrometer on the package, will

then give a reasonable idea of the device’s

junction temperature.

MOSFET Transient Thermal Issues

Having chosen a MOSFET that will withstand the imposed

voltage stresses, and the worse case continuous I2R power

dissipation which it will see, it remains only to verify the

MOSFET’s ability to handle short-term overload power dissi-

pation without overheating. A MOSFET can handle a much

higher pulsed power without damage than its continuous

dissipation ratings would imply. The reason for this is that, like

everything else, thermal devices (silicon die, lead frames,

etc.) have thermal inertia.

In terms related directly to the specification and use of power

MOSFETs, this is known as “transient thermal impedance,”

or Zθ(J-A). Almost all power MOSFET data sheets give a

Transient Thermal Impedance Curve. For example, take the

following case: VIN = 12V, tOCSLOW has been set to 100msec,

ILOAD(CONT. MAX) is 2.5A, the slow-trip threshold is 50mV

nominal, and the fast-trip threshold is 100mV. If the output is

accidentally connected to a 3Ω load, the output current from

the MOSFET will be regulated to 2.5A for 100ms (tOCSLOW)

before the part trips. During that time, the dissipation in the

MOSFET is given by:

P = E x I EMOSFET = [12V-(2.5A)(3Ω)] = 4.5V

PMOSFET = (4.5V x 2.5A) = 11.25W for 100msec.

At first glance, it would appear that a really hefty MOSFET is

required to withstand this sort of fault condition. This is where

the transient thermal impedance curves become very useful.

Figure 10 shows the curve for the Vishay (Siliconix) Si4410DY,

a commonly used SO-8 power MOSFET.

Taking the simplest case first, we’ll assume that once a fault

event such as the one in question occurs, it will be a long

time– 10 minutes or more– before the fault is isolated and the

channel is reset. In such a case, we can approximate this as

a “single pulse” event, that is to say, there’s no significant duty

cycle. Then, reading up from the X-axis at the point where

“Square Wave Pulse Duration” is equal to 0.1sec (=100msec),

we see that the Zθ(J-A) of this MOSFET to a highly infrequent

event of this duration is only 8% of its continuous Rθ(J-A).

This particular part is specified as having an Rθ(J-A) of

50°C/W for intervals of 10 seconds or less. Thus:

Assume TA = 55°C maximum, 1 square inch of copper at the

drain leads, no airflow.

Recalling from our previous approximation hint, the part has

an RON of (0.0335/2) = 17mΩ at 25°C.

Assume it has been carrying just about 2.5A for some time.

When performing this calculation, be sure to use the highest

anticipated ambient temperature (TA(MAX)) in which the

MOSFET will be operating as the starting temperature, and

find the operating junction temperature increase (∆TJ) from

that point. Then, as shown next, the final junction temperature

is found by adding TA(MAX) and ∆TJ. Since this is not a closed-

form equation, getting a close approximation may take one or

April 2003

19

MIC2582/MIC2583

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